@anoman @altimys
I find that Souboutei's... I guess "manner of operation" for lack of a better term, is really systematic, which makes sense considering who's doing most of the planning (a logic-based alien).
Like, I sort of see the entire Souboutei mansion as a "body" of sorts. First, everyone is attacked by the mindless hordes which should eliminate most threats right away, and they move automatically which means fewer resources are used up. They remind me of white blood cells or something. Next, the survivors are attacked by the paintings, which fight invading "cells" individually and adapting to them, sort of like antibodies. Then, once the invading group is culled significantly by this point, the more powerful possessed troops come out to finish the job and protect what's most important: the heart and brain (aka deido).
I also get the impression that Shino is carrying out like 20 plans at once, so whenever something goes awry they have like 5 fallback options for everything. That's my rationalization for why the invaders are doing a lot of random shit all the time. Like, for one thing every time they get an order from Deido they try their very best to undermine his power in any way they can, but also in their free time they've been digging random holes, surely not on Deido's orders, as well as going so far as possessing the corpses of Seiichi's old friends and taking them across planets. I think the main reason of "Why didn't the invaders do X" will always come down to "Deido didn't let them." If they had the willpower to defy Deido, that'd be another story, but ironically, the very thing that makes them so capable of brutality (not being human) is also the reason they can't resist Deido's control.